The present invention pertains to a process for the production of vinyltoluene, and more specifically to a method for inhibiting the formation of undesirable cross-linked thermal polymers in the manufacture of vinyltoluene.
Vinyltoluene is commonly prepared by the catalytic dehydrogenation of ethyltoluene, cooling to condense the gaseous materials and fractionally distilling the liquid product to obtain vinyltoluene.
In processes utilized in the past, the crude vinyltoluene produced by the catalytic dehydrogenation of ethyltoluene has been particularly susceptible to the formation of thermal polymers. When heated, the crude vinyltoluene cross-links to form an insoluble gelatinous polymer. This undesirable byproduct collects in the recycle column and in other parts of the condensation and distillation apparatus, necessitating the cleaning of the column and other equipment, with attendant work stoppage, delay, and expense.
In order to prevent the formation of the thermal polymer during distillation of vinyltoluene, various types of known polymerization inhibitors have been employed in connection with prior art distillation processes. These inhibitors have been only partially effective in preventing the formation of the undesirable thermal polymer in the distillation apparatus. In addition, since the inhibitor is added to the crude vinyltoluene product in the distillation apparatus, the inhibitor does not prevent the formation of the undesirable thermal polymers in the equipment used prior to distillation.
It has been discovered that the removal of hydrocarbons having more than 11 carbon atoms from the crude vinyltoluene greatly increases the time required for the crude vinyltoluene to cross-link, and hence, substantially lowers the amount of polymer formed during vinyltoluene production, particularly in the recycle column.